We have developed an in vivo selection system in which peptides that home selectively to different tissues are recovered after intravenous administration of a phage random peptide library. In earlier work, we uncovered a previously unrecognized address system that makes possible organ-specific targeting and angiogenesis-related targeting of tumors. The corresponding receptors for the peptides targeting tumors are cell surface markers that are upregulated or activated during tumor progression. We also developed methods for assessing organ-specificity and cell-type distribution of such probes in vivo. Our working hypothesis is that there are tissue-related and tumor stage related heterogeneity in the bladder; such diversity may affect the responses to therapies. Here, we propose to define the cellular and molecular differences that exist (i) in normal urothelium and (ii) in bladder cancer. Under Specific Aim #1, we will explore novel strategies based on in vivo and ex-vivo phage display combined with Laser Capture Microdissection (LCM) technologies. We aim to isolate peptides that home in vivo to the mouse bladder or that bind ex vivo to various bladder cell types. Under Specific Aim #2, we will isolate peptides and antibodies that home to the bladder at different stages of tumor progression, from early to late metastatic disease. In a complementary strategy, we will select peptides that are recognized by the circulating pool of immunoglobulins at selected stages of bladder cancer progression. These specific probes will be used to identify new tumor- and tumor-associated antigens. Under Specific Aim #3, we will isolate, clone, and evaluate the expression pattern and function of these specific and tumor-specific markers in the context of bladder cancer progression. The identification of novel bladder and bladder cancer receptors may shed light in the complex cellular and molecular diversity of the bladder. Under Specific Aim #4, we will identify specific peptides that have the ability to internalize into urothelial cells. The combination of tight cell junctions, a specialized luminal cell surface bearing glycosaminoglycans (GAG), and a mucin layer protect the mammalian urothelium against concentrated urine chemicals and microbial adhesion. However, under pathological conditions, such features of the urothelium become obstacles to effective delivery of therapeutic agents to the bladder. Herein we propose to pursue candidate peptides with the ability to internalize into urothelial cells to develop therapeutic targeting strategies for the treatment of bladder cancer.